A while back I was making 3-1/2" 12 slug loads and asked if anyone knew what pressure the 12ga "brass" (actually steel) would stick. I said I would post if I ever found out. Well, it seems the magic # is around 20-21,000psi, a bit higher than my guess of 15-18k.
Having them tested was a big help in the right direction.
My favorite load tested in at 5848ftlbs energy @ 18,400psi. I imagine I could get the pressure down a few K without too much sacrifice in energy.

Interesting information. May I ask where you had the testing done, and how much they charged? Aren't sammi specs for 12 gauge a lot lower than the pressure levels you mention? What kind of gun are you shooting your slug loads out of?

I had it done at:
Ballistic Research
1108 West May Ave
Mc Henry, IL 60050

The friendly gent that did the work is Tom Armbrust, 815-385-0037.
I sent him 11 different shells @ $5 per shell.

Yes, all but one exceeded the 14K psi SAAMI max, and that one was a 2-1/2oz shot load, it made 4835ftlbs of energy @ 12,100psi. I'd say a 50% increase over the most powerful factory magnum, yet at less pressure is a great success.
Oddly, the highest pressure shell tested was almost identical to the one above yet used 15% less of a one step hotter powder. It had basically the same velocity and power (4900ftlbs), but was more than double the pressure @ 25,700psi!
The hottest one I shot in my gun (Mossberg 835) tested at 20,400psi, and that's where they would stick.